In the process for silicon wafer production, there are a number of steps in which silicon wafers directly contacted and supported on their back sides by a supporting member are subjected to heat treatment. For example, a heat treatment step using a vertical boat, an RTA (rapid thermal annealing) step, a single-feed type epitaxial growth step and an SOI heat treatment step are relevant, and in such many heat treatments, silicon wafers directly contacted and supported on their back sides by a supporting member are subjected to various processes.
For example, in the heat treatment of silicon wafers in a vertical batch-wise heat treatment furnace, use is made of a wafer supporter (hereinafter referred to as “supporting boat”), as shown in FIG. 1, which supports each wafer at three or four points in an outer peripheral portion on the back of the wafer. Thus, the supporting boat 1 comprises three or four columns 3, a top end plate 5 and a bottom end plate 6 that respectively are attached to the upper and lower ends of the columns 3, and a sideways opening 2. Each of the columns 3 has a row of wafer-supporting elements 4 and, after placing silicon wafers on such supporting elements 4 through the sideways opening 2, the whole boat is inserted into a vertical heat treatment furnace for each intended heat treatment.
However, when silicon wafers are placed on such supporting boat and subjected to heat treatment, a problem is encountered; namely, crystal defects called slip initiating from the damage incurred at the point of contact between the wafer and each supporting element surface may appear in each silicon wafer, and such slip grows and develops under the influence of the thermal stress caused during heat treatment, with the result that a wafer yield is decreased. In the case of wafers having a diameter of 300 mm or larger, such slip growth and extension may also be caused by the stress resulting from the wafer's own weight (gravity stress).
As a means for solving such problem, there is known a wafer supporting jig in which an area of contact between each wafer and each supporting element is widened so that the load incurred by the wafer's own weight may be dispersed and the gravity stress may be reduced as much as possible.
However, a groove (corresponding to a gap between surfaces of two neighboring wafer supporting elements 4 in the supporting boat shown in FIG. 1) in this supporting jig should have different machining accuracy or induce different contact surface conditions (flatness and surface roughness) and, for some grooves, the stress incurred by wafer's own weight may not be dispersed all over the contact surface but may be concentrated at a specific position, for example single point, and this contact point may act as an initiation point for slip generation.
For avoiding the state such that the contact point may act as an initiation point for slip generation in the wafer supporting jig, control is made so that each wafer may come into contact with the whole relevant surface of each supporting element at each groove; however, this is actually difficult. Thus, the supporting jig having an increased wafer-supporting contact area cannot be said to exhibit a satisfactory slip inhibiting effect.
On the other hand, it is known with regard to silicon crystals that the stress contributing to slip extension varies depending on the crystal orientation. Japanese Patent Application Publication (Kokai) No. Hei 09-139352 discloses an invention concerning a wafer boat for a vertical furnace by which stress generation by wafer's own weight can be reduced; in this document, the calculation of critical shear stresses upon application of a predetermined thermal stress to a silicon wafer are made for twelve crystal slip systems governing slip generation in silicon wafers.
According to those results, along the crystal orientations <110> and <100>, slip generation hardly occurs, and it is concluded that the slip generation can be suppressed by supporting the wafer having crystalline (001) face in a direction of the crystal orientation <100> or <110> on its back side.
However, even if this technology is utilized to suppress slip generation, it is difficult to suppress slip growth using the above-mentioned wafer supporting jig having an increased wafer-supporting contact area in the support since the crystal orientation varies according to directions on the wafer surface and the stress (shear stress) contributing to slip extension varies accordingly and the wafer-supporting element contact point which acts as an initiation point for the slip cannot be singled out.
This problem of slip generation is not limited to a vertical batch-wise heat treatment furnace but common to processes in which wafers locally supported on their back sides are subjected to heat treatment, for example in a single-feed type heat treatment furnace or epitaxial growth treatment furnace.